1. Field of the Invention
This invention relates generally to a method for revising the current/voltage relationship of a fuel cell stack over time and, more particularly, to a method for revising the current/voltage relationship of a fuel cell stack as it ages that includes changing a first adaptation value when the stack is operating in a kinetic region and changing a second adaptation value when the stack is operating in an ohmic region.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The dynamic power of a fuel cell system is limited. Further, the time delay from system start-up to driveability and low acceleration of the vehicle may not be acceptable. The voltage cycles can decrease the stack durability. These drawbacks can be minimized by using a high voltage battery in parallel with the fuel cell stack. Algorithms are employed to provide the distribution of power from the battery and the fuel cell stack to meet the requested power.
For the reasons discussed above, some fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental power source in addition to the fuel cell stack, such as a DC battery or a super capacitor (also referred to as an ultra-capacitor or double layer capacitor). The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to a traction motor and other vehicle systems through a DC voltage bus line for vehicle operation. The battery provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery through the DC bus line.
FIG. 1 is a schematic block diagram of a hybrid fuel cell system 10 including a fuel cell stack 12 and a high voltage battery 14. The battery 14 is intended to represent any type of rechargeable energy storage system (RESS) suitable for the fuel cell application described herein. The fuel cell stack 12 provides electrical power to a high voltage bus line, represented here as positive bus line 16 and negative bus line 18. In a vehicle fuel cell system, the fuel cell stack 12 may include about 400 fuel cells. The battery 14 is also coupled to the high voltage bus lines 16 and 18, and provides supplemental power as discussed above. A power controller 20 controls the distribution of power provided by the stack 12 and the battery 14.
The fuel cell system 10 includes a power inverter module (PIM) 22 electrically coupled to the bus lines 16 and 18 and an AC or DC traction motor 24. The PIM 22 converts the DC voltage on the bus lines to an AC voltage suitable for the AC traction motor 24. The traction motor 24 provides the traction power to operate the vehicle, as is well understood in the art. The traction motor 24 can be any suitable motor for the purposes described herein, such as an AC induction motor, an AC permanent magnet motor and an AC three-phase synchronous machine. During regenerative braking when the traction motor 24 is operating as a generator, electrical AC power from the motor 24 is converted to DC power by the PIM 22, which is then applied to the bus lines 16 and 18 to recharge the battery 14. A blocking diode (not shown) prevents the regenerative electrical energy applied to the bus lines 16 and 18 from flowing into the fuel cell stack 12, which could otherwise damage the stack 12.
For a typical hybrid vehicle strategy, the battery 14 is mainly used to increase efficiency, lower the dynamic requirements of the fuel cell system, and/or increase the performance of the vehicle. If the vehicle operator demands more power, the battery 14 can provide the stored power to the traction motor 24 very fast. Additional demanded power can be quickly provided by the fuel cell system.
It is necessary to determine how much current can be drawn from the fuel cell stack 12 and the battery 14 as the load on the system 10 changes. To do this, the power controller 20 needs to know how the current draw will split between the fuel cell stack 12 and the battery 14 so that more current is not drawn from the stack 12 or the battery 14 than it is able to provide. The power controller 20 is able to predict the current split between the fuel cell stack 12 and the battery 14 if it can predict their electrical behavior. For example, if the stack 12 can provide 500 A and the battery 14 can provide 200 A, the controller 20 needs to limit the total current draw from both the stack 12 and the battery 14 consistent with their actual ability to provide the current. This may result in limiting the stack current to 400 A. A current split of one-third to two-thirds may be predicted by the power controller 20 in order to not exceed the battery capability.
In the system 10, the fuel cell stack 12 and the battery 14 are directly coupled to the high voltage bus lines 16 and 18. However, this is merely a representative example of a fuel cell system for a hybrid vehicle. Ultra-capacitors and DC/DC converters can also be employed to match the different voltage ranges of the fuel cell stack 12 and the battery 14, as would be well understood to those skilled in the art. For example, known fuel cell hybrid vehicles sometimes employ a bidirectional DC/DC converter to step up the DC voltage from the battery to match the battery voltage to the bus line voltage dictated by the stack voltage and step down the stack voltage during battery recharging. Various designs have been proposed in the art to eliminate the DC/DC converter because it is large, costly and heavy.
The controller 20 needs to know the current/voltage relationship, referred to as a polarization curve, of the fuel cell stack 12 and the battery 14 to provide a proper distribution of power. The relationship between the current and the voltage for the battery 14 is well modeled across its operating range. The relationship between the voltage and the current of the stack 12 is more difficult to define because it is non-linear, and changes depending on stack temperature, partial pressures and cathode and anode stoichiometries. Additionally the relationship between the stack current and voltage changes as the stack degrades over time. Particularly, an old and degraded stack will have lower cell voltages, and will need to provide more current to meet the power demands than a new, non-degraded stack.
When a vehicle operator makes a torque or power request to the fuel cell system 10, the power controller 20 converts the power request to a current demand for the fuel cell stack 12. FIG. 2 is a graph with stack current on the horizontal axis and stack voltage on the vertical axis showing a polarization curve 36 for a new fuel cell stack and a polarization curve 38 for an old fuel cell. If the vehicle operator requests 48 kW of power from the fuel cell system 10, the power controller 20 will determine how much current the stack 12 needs to provide based on the voltage that the stack is currently able to produce for that power request, and set the compressor speed and the hydrogen flow accordingly for that current. In this example, the new stack produces about 320 volts, which requires a stack current of about 150 amps. However, for the old stack, which is only able to generate about 290 volts, the compressor speed and the hydrogen flow would need to be set for about 163 amps of stack current to meet the 48 kW of requested power.
In some fuel cell system designs, a voltage source and an internal resistance of the source are used to model the dependency between stack voltage and current. The resistance value and the idle voltage of the voltage source need to be continuously changed in order to adapt to the actual voltage/current relationship of the stack as it ages. Therefore, two parameters have to be adapted based on only one actual pair of current and voltage, which leads to an infinite number of solutions. To address this problem, the known power algorithms slow down the adaptation of the parameters, which leads to using the actual current/voltage pair as well as pairs of recent time steps.